1. Field of the Invention
The present invention relates to an electric power tool having an output control structure.
2. Related Art
An electric power tool such as an impact driver has a switching structure, which enables the tool to be operated in the alternative of two modes in output in accordance with material to be worked and a finishing operation applied thereto, in order to prevent a head of a screw to be wrenched away from it shank or a screw to be turned excessively tightly, as shown in Japanese Patent Provisional Publication No. H7-326249.
More specifically, such a kind of electric power tool has a trigger, which is connected to a switch box so that output increases in proportion to an amount of pulling operation of the trigger, and a dial type-adjustment knob is normally placed between the trigger and the switch box so that turning the knob between two positions enables the tool to be operated in the alternative of two modes in the maximum amount of pulling operation of the trigger to provide a controlled output. Turning the knob to one of the two modes, i.e., the low output mode enables the output to be increased to the maximum in the low output mode in proportion to an amount of pulling operation of the trigger. Turning the knob to another mode, i.e., the high output mode enables the trigger to be further pulled to increase the output to the maximum in the high output mode in proportion to an amount of pulling operation of the trigger.
There has been another type of electric power tool provided with output terminals extended from a switch box. In such a tool, a signal for switching the rotation mode is detected by an independent switch to make a change in output voltage and number of revolutions. The switching operation of the above-mentioned independent switch between high and low modes makes it possible to make a change in degree of variation of output in accordance with an amount of pulling operation of the trigger.
However, in the former conventional power tool, when the trigger is pulled, an operator's finger may come into contact with the knob, thus compelling an operator to work under a load, especially in case where a long period of working time is required. Fingers may come into contact with the knob to turn it inadvertently, resulting in occurrence of a problem of change in number of revolutions of an output shaft. In addition, in the former conventional power tool in which the amount of pulling operation of the trigger is switched by the knob, variations in an amount of pulling operation of the trigger occur due to variation in thickness of the knob and variation in clearance between the knob and the housing. As a result, when output characteristics are provided based on a proportional curve, errors in output voltage increase in accordance with variation in the amount of pulling operation of the trigger. Accordingly, a problem may occur that, when the low output is actually required, a screw is turned on a high output power.
In the latter conventional power tool, modification of a circuit in the general-purpose switch box is required, thus resulting in increased costs of parts. In addition, the switch for detecting the signal must be provided independently, thus increasing the number of parts and leading to complicated handling of lead wires. Accordingly, the assembling property is deteriorated.